Process for the oxidation of alcohols to aldehydes

ABSTRACT

A process for the preparation of an aldehyde which comprises reacting the corresponding alkanol with a solubilized stable free radical nitroxide having the formula:   &lt;IMAGE&gt;   wherein each of R1, R2, R3 and R4 is an alkyl, aryl or heteroatom substituted alkyl group having 1 to about 15 carbon atoms and each of R5 and R6 is alkyl, hydrogen, aryl or a substituted heteroatom, an alkali metal nitrosodisulfonate, a non-basic polar solvent and an oxidant, for about eight hours or less at a temperature in the range of from about 10 DEG  C. to about 20 DEG  C. and thereafter separating out the aldehyde.

FIELD OF THE INVENTION

This invention relates to a process for the preparation of aldehydes bythe oxidation of the corresponding alcohols in the presence of a stablefree radical nitroxide, an alkali metal nitrosodisulfonate, a non-basicpolar solvent and an oxidant.

BACKGROUND OF THE INVENTION

It is known to use nitroxyl radicals/oxoammonium salts in the oxidationof primary alcohols to produce aldehydes and acids and the oxidation ofsecondary alcohols to produce ketones (Journal of Organic Chemistry,vol. 52 (12), pp. 2559-2562 and Journal of Organic Chemistry, vol. 55,1990, pp. 462-466).

It is reported in the open literature that primary aliphatic alcoholscan be converted to aldehydes, but only in 30-40% yields in the presenceof catalytic amounts of cuprous chloride,2,2,6,6,-tetramethylpiperidine-1-oxyl, and atmoshperic oxygen (Journalof American Chemical Society, 1984, 106 pp. 3374). It is also known thathigher yields of aldehydes can be obtained if stoichiometric amounts ofcupric or ferric salts are used instead of catalytic amounts of thecuprous salts (Pure and Applied Chemistry, vol. 62(2), 1990, pp.217-222).

OBJECTS OF THE INVENTION

It is an object of this invention to produce aldehydes with highselectivities at moderate conversions from alkanols without producinglarge amounts of other products such as acids and esters.

It has been found that aldehydes can be produced with high selectivitesand moderate conversions by using catalytic amounts of a stable freeradical nitroxide, an alkali metal nitrosodisulfonate and an oxidant.

SUMMARY OF THE INVENTION

This invention relates to a process for the preparation of an aldehydewhich comprises reacting the corresponding alkanol with a solubilizedstable free radical nitroxide having the formula: ##STR2## wherein eachof R₁, R₂, R₃ and R₄ is an alkyl, aryl or heteroatom substituted alkylgroup having 1 to about 15 carbon atoms and each of R₅ and R₆ is alkyl,hydrogen, aryl or a substituted heteroatom, an alkali metalnitrosodisulfonate, a non-basic polar solvent and an oxidant at atemperature in the range of from about -10° C. to about 20° C. for abouteight hours or less, and thereafter separating out the aldehyde.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present process converts alkanols to the corresponding aldehydes bycontacting the alkanol with a solubilized stable free radical nitroxide,an alkali metal nitrosodisulfonate, a non-basic polar solvent and anoxidant, for about eight hours or less at a temperature in the range offrom about -10° C. to about 20° C.

The alkanol reactant suitably comprises one or more alkanols having acarbon number in the range of from about 1 to about 45. An alkanolconsisting essentially of primary, mono-alkanols is preferred. Mostpreferably, the alkanol reactant consists essentially of one or more C₆to C₃₀ primary mono-alkanols. Preference can also be expressed foralkanols having from 8 to about 20 carbon atoms, with C₉ to C₁₈ alkanolsconsidered more preferred and C₁₁ to C₁₈ alkanols considered mostpreferred. As a general rule, the carbon chains of the alkanols may beof either branched or linear (straight-chain) structure, althoughpreference further exists for alkanol reactants in which greater thanabout 50 percent, more preferably greater than about 70 percent and mostpreferably greater than about 90 percent of the molecules are of linear(straight-chain) carbon structure. In large part, such preferencesrelate more to the utility and value of the products than to theoperability or performance of the process of the invention.

The general suitability of such alkanols as reactants in oxidationreactions is well recognized in the art. Examples of specific alkanolsand of commercially available alkanols and alkanol mixtures within thisclass are also well known.

Examples of commercially available alkanol mixtures include the NEODOLAlcohols, trademark of and sold by Shell Chemical Company, includingmixtures of C₉, C₁₀ and C₁₁ alkanols (NEODOL 91 Alcohol), mixtures ofC₁₂ and C₁₃ alkanols (NEODOL 23 Alcohol), mixtures of C₁₂, C₁₃, C₁₄, andC₁₅ alkanols (NEODOL 25 Alcohol), and mixtures of C₁₄ and C₁₅ alkanols(NEODOL 45 Alcohol); the ALFOL Alcohols, trademark of and sold by VistaChemical Company, including mixtures of C₁₀ and C₁₂ alkanols (ALFOL1012), mixtures of C₁₂ and C₁₄ alkanols (ALFOL 1214), mixtures of C₁₆and C₁₈ alkanols (ALFOL 1618), and mixtures of C₁₆, C₁₈ and C₂₀ alkanols(ALFOL 1620); the EPAL Alcohols, trademark of and sold by Ethyl ChemicalCompany, including mixtures of C₁₀ and C₁₂ alkanols (EPAL 1012),mixtures of C₁₂ and C₁₄ alkanols (EPAL 1214), and mixtures of C₁₄, C₁₆,and C₁₈ alkanols (EPAL 1418); and the TERGITOL-L Alcohols, trademark ofand sold by Union Carbide Corporation, including mixtures of C₁₂, C₁₃,C₁₄, and C₁₅ alkanols (TERGITOL-L 125). Also very suitable are thecommercially available alkanols prepared by the reduction of naturallyoccurring fatty esters, for example, the CO and TA products of Procterand Gamble Company and the TA alcohols of Ashland Oil Company.

The term "stable free radical nitroxide" as used herein shall mean afree radical nitroxide that can be prepared by conventional chemicalmethods and will exist long enough to be used in a subsequent chemicalreaction or examined in a static system by normal methods ofspectroscopy. Generally, the stable free radical nitroxides of thepresent invention have a half life of at least one year. The term"stable free radical" shall also be understood to include the presursorto a stable free radical from which the stable free radical may beproduced in situ.

The stable free radical nitroxides, as used in the present process, areprecursors to catalysts, i.e., oxoammonium salts, active for theoxidation of alkanols to the corresponding aldehydes. These catalystsare generated in situ by the oxidation of a stable free radicalnitroxide to an oxoammonium salt with an oxygen-containing oxidant. Thestable free radical nitroxide can be obtained by the oxidation ofsecondary amines or hydroxylamines.

The stable free radical nitroxides which are suitable for use in theinstant invention have the formula: ##STR3## wherein each of R₁, R₂, R₃and R₄ is an alkyl, aryl or heteroatom substituted alkyl group having 1to about 15 carbon atoms, and no hydrogen is bound to the remainingvalences on the carbon atoms bound to the nitrogen, and each of R₅ andR₆ is alkyl, hydrogen, aryl or a substituted heteroatom. As used herein,the term "alkyl" is meant to include cycloalkyl. The alkyl (orheteroatom substituted) groups R₁ -R₄ may be the same or different, andpreferably contain 1 to 15 carbon atoms. Preferably, R₁ -R₄ are methyl,ethyl, or propyl groups. In addition to hydrogen, the heteroatomsubstituents may include, halogen, oxygen, nitrogen and the like.Preferably, one of R₅ and R₆ is hydrogen while the other is asubstituted heteroatom which does not interfere with the reaction.Suitable substituted heteroatoms include, --OR, ##STR4## --O--SO₃ H,--O--polymer and the like.

In a preferred embodiment, the nitroxide is selected from the groupconsisting of 2,2,6,6-tetramethyl-piperidine-1-oxyl,4-hydroxy-2,2,6,6-tetramethyl-piperidine-1-oxyl,4-oxo-2,2,6,6-tetramethyl-piperidine-1-oxyl,2,2,6,6-tetramethyl-piperidine-1-oxyl-4-sulfate,4-alkoxy-2,2,6,6-tetramethyl-piperidine-1-oxyl and mixtures thereof,with 2,2,6,6-tetramethyl-piperidine-1-oxyl,2,2,6,6-tetramethyl-piperidine-1-oxyl-4-sulfate, and4-alkoxy-2,2,6,6-tetramethyl-piperidine-1-oxyl being particularlypreferred.

The alkali metal nitrosodisulfonate reactant may suitably be any alkalimetal nitrosodisulfonate although potassium nitrosodisulfonate ispreferred. As used herein, the term "alkali metal" is used as adescriptor of the elements Group IA of the Periodic Table of theElements (Li, Na, K, Rb, Cs, Fr). The alkali metal nitrosodisulfonatecan be added to water prior to being added to the reaction mixture,although is typically added as a solid after all of the other reactantshave been added. While not wishing to be bound by any particular theory,it is believed that the alkali metal nitrosodisulfonate decomposes underthe reaction conditions, and one or more of these decomposition productsappears, in the presence of an oxidant, to become an oxidant which iscapable of oxidizing the nitroxide to an oxoammonium salt. It isbelieved that nitrogen oxides (NO_(x)) are generated in the reaction andare the active species.

The oxidants suitable for use in the instant invention are thosecompounds which are capable of oxidizing the stable free radicalnitroxide to the oxoammonium salt. Suitable oxidants include oxygen oran oxygen-containing gas such as air. Whereas pure oxygen is preferredto accomplish the desired conversion, the oxygen can be diluted with aninert gas such as nitrogen, helium, argon, or other similar gas. Whileair can be used as the oxidant, the reaction rate is much slower. Forpurposes of increasing the reaction rate, higher O₂ pressures such as,for example, 1000 psi can be utilized. In a preferred embodiment, pureoxygen is used as the oxidant and it is bubbled into the reactionsolution. In another embodiment, oxygen can be bubbled initially throughthe reaction solution in order to commence the reaction and then theflow of oxygen can be stopped without stopping the reaction.

The amounts and concentrations of the reactants utilized in the processof the instant invention can vary widely. The amount of stable freeradical nitroxide is typically in the range of from about 1 mole percentto about 50 mole percent, preferably from about 5 mole percent to about20 mole percent, basis the weight number of moles of the startingalkanol. Generally, the amount of an alkali metal nitrosodisulfonateutilized will be in the range of from about 1 mole percent to about 35mole percent, preferably from about 10 mole percent to about 20 molepercent, basis the number of moles of starting alkanol.

The reaction in the instant invention is carried out utilizing asolubilized stable free radical nitroxide. The solvent utilized is anon-basic polar solvent, i.e., a solvent which does not form a strongcomplex with the oxoammonium ion, in which the alkanol is readilysoluble. While the reaction will proceed if non polar solvents areutilized, the reaction times are so much slower that typically less than10% conversions are obtained, even with extended reaction times. Whenbasic polar solvents are utilized in the reaction, the reaction proceedsat an extremely slow rate and significant amounts of aldehydes are notproduced. Non-basic polar solvents which are most suitable are thosehaving dielectric constants less than about 20 and those solvents whichare inert in the reaction. The solvent may be added to the reactionmixture or, alternatively, the nitroxide may be dissolved in the solventprior to addition of the alkali metal nitrosodisulfonate to the reactionmedium. The solvent is typically selected from the group consisting ofwith acetonitrile, dimethylsulfolane, nitroethane, and mixtures thereof,with acetonitrile being preferred. The amount of solvent utilized in theprocess is generally from about 20:1 to about 0.5:1, preferably fromabout 10:1 to about 5:1, basis the weight of the starting alkanol.

The process of the present invention is typically conducted under mildconditions, with good results being obtained using a temperature in therange of from about -10° C. to about 20° C., preferably from about 0° C.to about 20° C. and more preferably from about 10° C. to about 15° C.Reaction pressures are not critical although higher pressures result inincreased reaction rates. Pressures in the range of from aboutatmospheric pressure up to about 1000 psig can be employed with goodresults. The time of reaction required in order to obtain a highselectivity to aldehydes is typically about eight hours or less. Thetime in which the reaction proceeds to aldehydes will, however, belonger in the event that non polar solvents such as dichloromethane,carbon tetrachloride and heptane are used. The optimum times andtemperatures for maximizing the selectivity to aldehydes can be readilydetermined by one skilled in the art with a minimal amount of routineexperimentation.

The process of the instant invention can be carried out either batchwiseor continuously, using a stirrer equipped reactor or other well knowncontacting technique to achieve adequate mixing. Preferred reactionconditions, e.g., temperature, pressure, flow rates, etc., vary somewhatdepending on the specific nitroxide utilized and on the concentration ofthe nitroxide.

The process of the instant invention can be carried out in a variety ofways. For example, 0.032 moles of alkanol, 0.006 moles of the nitroxide,0.0075 moles of an alkali metal nitrosodisulfonate which has beendissolved in water and solvent may be added to the reaction vessel,followed by bubbling an O₂ stream through the reaction mixture.Alternatively, the alkanol, the alkali metal nitrosodisulfonate andsolvent may be added to the reaction vessel and allowed to reachequilibrium, followed by the dropwise or immediate addition of 10 molepercent of nitroxide which has been dissolved in a minimum amount ofsolvent. In a preferred embodiment, the reaction is carried out byadding the alkanol, the nitroxide, the solvent and the alkali metalnitrosodisulfonate together and then bubbling an oxidizing gas throughthe mixture. Following the reaction, the product may be separated fromthe reaction mixture using conventional procedures such as extractionusing a suitable extraction solvent such as, for example, ethyl acetate;evaporation wherein the solvent is stripped from the reaction mixture byusing heat or vacuum. The reaction product can be purified by a numberof conventional means such for example, as distillation.

Depending upon process conditions and the nitroxide used, theselectivity to aldehyde obtained by this invention can be greater thanabout 65%. The products produced by the instant process can be used in avariety of applications. For example, these products are typically usedas or as intermediates in the preparation of esters, imides, acids,imines and amines.

The ranges and limitations provided in the instant specification andclaims are those which are believed to particularly point out anddistinctly claim the present invention. It is, however, understood thatother ranges and limitations which perform substantially the samefunction in the same or substantially the same manner to obtain the sameor substantially the same result are intended to be within the scope ofthe instant invention as defined by the instant specification andclaims.

The process of this invention will be further described by the followingembodiments which are provided for illustration and are not to beconstrued as limiting the invention.

Illustrative Embodiments EXAMPLE 1

6 Grams of 1-dodecanol, 1 gram of 2,2,6,6-tetramethyl-piperidine-1-oxyl,50 milliliters of acetonitrile, 2 milliliters of water and 2 grams ofpotassium nitrosodisulfonate were charged to a 100 milliliter roundbottomed flask. An O₂ stream was then bubbled through this mixture atatmospheric pressure. The reaction temperature was held at 10° C. overan 8 hour period. The results are presented in Table I.

EXAMPLE 2

6 Grams of 1-dodecanol, 1 gram of 2,2,6,6-tetramethylpiperidine-1-oxyl,50 milliliters of acetonitrile, 2 milliliters of water and 1.0 grams ofpotassium nitrosodisulfonate were charged to a 100 milliliter roundbottomed flask. An O₂ stream was then bubbled through this mixture atatmospheric pressure. The reaction temperature was held at 15° C. over a4 hour period. The results are presented in Table I.

COMPARATIVE EXAMPLE A

Comparative Example A was carried out in a manner similar to Example 1except that the reaction was run at 35° C. The results are presented inTable I.

COMPARATIVE EXAMPLE B

Comparative Example B was carried out in a manner similar to Example 1,except that the reaction was performed using heptane instead ofacetonitrile and the reaction was held at 20° C. over a sixteen hourperiod. The results are presented in Table I.

As can be seen in Table I, reaction temperatures exceeding 20° C. resultin the formation of carboxylic acids rather than the desired aldehydes.In addition, it can be seen that conversion to aldehydes is much slowerin non polar solvents.

                  TABLE I                                                         ______________________________________                                        Oxidation Of Alkanols to Aldehydes                                                                      % Sel.                                                     %        % Sel.    Esters +  % Sel.                                           Conversion                                                                             Aldehydes Heavies   Acids                                     ______________________________________                                        Example 1                                                                              59         97        3        0                                      Example 2                                                                              49         78        7       15                                      Comparative                                                                            92         19        8       73                                      Example A                                                                     Comparative                                                                              4.5      100       0        0                                      Example B                                                                     ______________________________________                                    

What is claimed is:
 1. A process for the preparation of an aldehydewhich comprises reacting the corresponding alkanol having a carbonnumber in the range of from about 1 to about 45 with a solubilizedstable free radical nitroxide having the formula: ##STR5## wherein eachof R₁, R₂, R₃ and R₄ is an alkyl, aryl or heteroatom substituted alkylgroup having 1 to about 15 carbon atoms and each of R₅ and R₆ is alkyl,hydrogen, aryl or a substituted heteroatom, an alkali metalnitrosodisulfonate selected from the group consisting of potassiumnitrosodisulfonate, sodium nitrosodisulfate and mixtures thereof, anon-basic polar solvent selected from the group consisting ofacetonitrile, dimethylsulfolane, nitroethane and mixtures thereof, andan oxygen-containing gas, for about eight hours or less at a temperaturein the range of from about 10° C. to about 20° C. and thereafterseparating out the aldehyde.
 2. The process of claim 1 wherein thesolubilized stable free radical nitroxide is selected from the groupconsisting of 2,2,6,6-tetramethyl-piperidine-1-oxyl,4-hydroxy-2,2,6,6-tetramethyl-piperidine-1-oxyl,4-oxo-2,2,6,6-tetramethyl-piperidine-1-oxyl,2,2,6,6-tetramethyl-piperidine-1-oxyl-4-sulfate,4-alkoxy-2,2,6,6-tetramethyl-piperidine-1-oxyl and mixtures thereof. 3.The process of claim 2 wherein the solubilized stable free radicalnitroxide is selected from the group consisting of2,2,6,6-tetramethyl-piperidine-1-oxyl,2,2,6,6-tetramethyl-piperidine-1-oxyl-4-sulfate,4-alkoxy-2,2,6,6-tetramethyl-piperidine-1-oxyl and mixtures thereof. 4.The process of claim 1 wherein said non-basic polar solvent isacetonitrile.
 5. The process of claim 1 wherein said alkali metalnitrosodisulfonate is potassium nitrosodisulfonate.
 6. The process ofclaim 1 wherein said alkanol is contacted with said solubilized stablefree radical nitroxide and said alkali metal nitrosodisulfonate,followed by the addition thereto of said oxidant.
 7. The process ofclaim 6 wherein the amount of solubilized stable free radical nitroxideis in the range of from about 1 mole percent to about 50 mole percent,basis the weight of the alkanol.
 8. The process of claim 7 wherein theamount of said alkalimetal nitrosodisulfonate is in the range of fromabout 1 mole percent to about 35 mole percent, basis the number of molesof said alkanol.
 9. The process of claim 1 wherein said alkanol iscontacted with said alkali metal nitrosodisulfonate, followed by theaddition thereto of said stable free radical nitroxide.
 10. The processof claim 9 wherein the amount of solubilized stable free radicalnitroxide is in the range of from about 1 mole percent to about 50 molepercent, basis the number of moles of said alkanol.
 11. The process ofclaim 10 wherein the amount of said alkalimetal nitrosodisulfonate is inthe range of from about 1 mole percent to about 35 mole percent, basisthe number of moles of said alkanol.
 12. The process of claim 1 whereinsaid oxygen-containing gas is selected from the group consisting of pureoxygen and air.
 13. The process of claim 12 wherein saidoxygen-containing gas is pure oxygen.
 14. The process of claim 1 whereinsaid process is carried out at a temperature in the range of from about0° C. to about 20° C. and at atmospheric pressure.
 15. The process ofclaim 14 wherein said process is carried out at a temperature in therange of from about 10° C. to about 15° C. and at atmospheric pressure.